Route guidance systems are heretofore known which perform route guidance to allow movement between lanes. For example, JP2007-127416A discloses a route guidance system configured to retrieve a retrieval route, set a guidance point on the basis of the retrieval route, set a standard route guidance point for the guidance point, and determine whether a lane change prohibited zone is present on the retrieval route from an initial route guidance point based on the standard route guidance point to the guidance point. The lane change prohibited zone is a zone in which movement between lanes is prohibited. When the lane change prohibited zone is present, the route guidance point is set at the nearer side by a value to which the length of the lane change prohibited zone is added. When the vehicle arrives at the route guidance point, the route guidance system performs a voice outputting process to output route guidance in voices with regard to an intersection to be guided.
When the above technique of JP2007-127416A is applied to a vehicle and the vehicle changes lanes in accordance with the route guidance, an actual lane change may be performed in accordance with surrounding traffic situations and the like. According to the above technique of JP2007-127416A, however, the route guidance point is set only on the basis of the length of the lane change prohibited zone. When a lane change is performed in accordance with the route guidance of JP2007-127416A, therefore, it is difficult for a vehicle to detect environmental changes on the guided route and an appropriate lane change cannot be executed.
A problem to be solved by the present invention is to provide a drive assist device that can execute appropriate lane changes.